I made a TNT cannon. The propellant TNT dropped into the water. I then primed the projectile TNT to shoot it. When the 'gunpowder' exploded, the 'bullet' shot as per normal. However, the cannon was a pile of dropped blocks.
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Confirmed. The same problem for me when using the 14w06b. With 14w05b everything works as "normal".
The bullet TNT is fired as normal but the TNT blocks in the water explode and break everything around.

As Freddy said, in 14w05b everything works "normally", but in 14w06b things get blown up.